Cropping photos for print

In a previous article I looked at how to resize images for web display; now it’s time to consider how to  size photos appropriately for printing, specifically how to ensure that you can print onto a given size of paper without stretching or squashing your image, or having to accept uneven borders.

It seems a simple question, but I have to confess that it was one that had me stumped for quite a while when I started to print digital images on a home inkjet. I tried all manner of programs but they didn’t seem to work. Later, when I started using Lightroom, I purchased a very good book which devotes around 30 pages to the print module. Even this weighty tome failed to mention the important point, which is:

The ratio of your image width to image height must match the ratio of the paper  width to height.

It seems so obvious now that I’m almost embarrased to admit that it took me a while to understand. But in my defence, I’d like to point out that camera designers and print manufacturers have done little to harmonise the dimensions of their sensors to match common print sizes – even though sometimes the same companies produce both products.

“Full frame” digital cameras have sensors measuring 24mm * 36mm (the same size as 35mm film). The number of pixels will vary from one model to another, but the ratio of the long side to the short side will always be 1.5:1. This can also be expressed as a 3:2 ratio. For example, the Canon EOS5D Mark II has a sensor with 3744 * 5616 pixels, approximately 21 megapixels. The ratio is 5616/3744 = 1.5

“APS-C” digital SLRs – the vast majority of digital SLRs on the market today –  also have this 1:1.5 ratio. The physical size of the sensor is smaller than the full-frame cameras, but the ratio of the width to height is the same. For example, the Canon EOS450D has 2848 * 4272 pixels, about 12.2 megapixels. The ratio is 4272/2847 = 1.5 again.

Digital compacts usually have  a ratio of 1.33

So now we know the most common aspect ratios for digital sensors:

Full frame and APS-C = 1.15 or 3:2
Compacts = 1.33 or 4:3

We can compare this to the sizes of paper which are commonly available for printing digital images. If you’re printing at home your likely choices are 6″ * 4″, 5″ * 7″, A4, and A3, in increasing order of size. If you’re sending your images to a commercial printer then a wider choice is available.

Taking the smallest size, 6″ * 4″ paper has an aspect ratio of 1:1.5 or 3:2. So a print from a full frame or APS-C camera will print onto the paper without any cropping. An image from a digital compact cannot be printed without cropping or uneven margins.

5″ * 7″ paper has an aspect ratio of 1.4 – so it’s not an ideal shape for printing from any digital camera.

A4 paper (the dimensions are 210mm * 297mm) has an aspect ratio of 1.41. Not ideal again.A3 paper is twice the area of A4 but the aspect ratio is exactly the same as A4.

The following table expands this analysis to a wider range of sizes, with the blue rows showing the paper sizes which are a good match for uncropped images from full-frame or APS-C DSLRs and the yellow rows showing the good matches for uncropped images from compact cameras. I’ve taken the print sizes from those offered by two popular commercial printers.

Target print size in inches			Print size ratio
Short Edge	Long Edge		long:short	short:long
4	6		1.5	2:3
5	7		1.4	5:7
8	10		1.25	4:5
8	12		1.5	2:3
8.3	11.7	(A4)	1.41	8.3:11.7
11	14		1.27	11:14
11.7	16.5	(A3)	1.41	8.3:11.7
12	16		1.33	3:4
16	20		1.25	4:5
20	30		1.5	2:3
24	36		1.5	3:4
30	40		1.33	3:4
36	48		1.33	3:4
40	60		1.5	2:3

Whilst it’s handy to know what paper sizes are a good match for uncropped images, it would be highly limiting to stick to those sizes, and in any case you will often want to crop your images to suit the subject matter.

Therefore it is very helpful to know how to crop your images to match any paper size.

I will show how to do this using two software options, Adobe Lightroom and GIMP.

Cropping for print using adobe lightroom

I’m assuming here that you’re already reasonably familiar with Lightroom so I’m not covering every single detail here.

It’s a good idea to create a “virtual copy” of your image before you start cropping. Although Lightroom will allow you to undo any changes, it’s handy to be able to view two (or more) versions of the image side by side so you can decide which you prefer.

The “Photo” menu includes the “Create Virtual Copy” command:

 Here we see the original image alongside the virtual copy, in Grid View:

Once you have your chosen photo selected in any Lightroom module, you can press “R” which is a shortcut to the crop option in the Develop module.

Above, you can see the options for the Crop tool. Notice the row that includes “Aspect:”, “Original”, and the padlock icon. When the padlock is closed, the crop is constrained to either the same aspect ratio as the original image, or a selected alternative ratio. When the padlock is clicked to make it open, any crop can be selected. For our purposes, we need to leave it closed.

Cick on the arrow next to “Original” to see the available choices:

There are a number of options provided with Lightroom, which includes some paper dimensions such as 4*6 and 5*7. Then underneath the row “Enter Custom ..” are custom aspect ratios which I have entered. In this example, we are going to crop the photo to fit A4 paper, which has dimensions of 210mm * 297mm.

Click “Enter Custom” and this dialog box appears:

Here I have entered the numbers 21.00 * 29.7 to represent the A4 aspect ratio. In later versions of Lightroom, you can enter up to 4 whole numbers in each box, so you wouldn’t need to change the millimetres into centimetres as I’ve done here, but the result is the same.

Once you press enter you will be presented with a crop box of the requested dimensions which you move about as you wish:

And after pressing Enter to confirm the crop:

The next time you do this, your custom crop will appear in the list of available aspect ratios:

You can create as many custom aspect ratios as you wish, to suit different paper sizes.

The next step in printing would be to go to the Print Module; this is well covered in other material available on-line or in books so I don’t intend to cover it here.

Cropping for print using Gimp

Assuming you have opened your chosen image in GIMP, then the shortcut to open the Crop tool is SHIFT+C. Pressing “C” by itself is the shortcut for the Clone tool, so don’t forget to hold down SHIFT while pressing C to get the Crop tool.

Above you can see that I have opened an image in GIMP and started the Crop tool. The Crop tool has a number of options and the important ones for our purpose are shown below.

If the checkbox to the left of “Fixed” is not already selected, select it now. The numbers shown below that are the current aspect ratio, expressed using the actual numbers of pixels in this image. You can see that it’s close to a square (1:1 ratio) and we need to change so that it can be printed to an A4 page (the same ratio applies to A3).

We know from the table above that the ratio we need is 1.4:1, so enter that value in tthe box:

Well, actually I entered the ratio the other way round which suits a “portrait” style crop where the longest edge is the vertical edge.  If you find that you’ve entered the aspect ratio the wrong way round, you can change it with the two small icons to the right which select portrait or landscape format.

The next step is to drag the crop boundary to where you want it to be. How you crop is up to you but here’s what it looked like in my example:

Note that because you have chosen a fixed aspect ratio, the possible crops are constrained. Here is the image after pressing the Enter key:

I think you can see that this particular image is not really ideal for printing to A4 paper because the required crop is too narrow to include all of the circular centre of the flower. I could have chosen another image to demonstrate, but this example shows that you don’t always have a totally free hand in choosing a paper size.

So how big can I print my image ?

Everything covered in this post so far is just about getting the image in the right proportions to print – we still need to consider whether we have enough pixels to get an acceptable print in a particular size.

That depends upon the resolution at which you print, expressed in DPI – dots per inch – although it makes more sense to think of Pixels Per Inch.

A commonly quoted resolution to aim for is 300 dpi. If the longest edge of your image is 3600 pixels, then you can print on a paper with a long edge of up to 12 inches. (3600 / 300 = 12).

In years gone by, people used to use a resolution of 240 dpi. This was considered a good resolution to use a few years ago. The reason that many people now advise the higher resolution of 300, is mainly because the increasing size of sensors means that they can, rather than any intrinsic shift in the value of the human eye to evaluate a printed image. My suggestion would be to use 300 dpi if you can, but if that won’t allow you to print at the size you want, then use 240 dpi. By the way, 300 and 240 are not “magic numbers” and you can print at 267, 283, or any other number.

The type of paper (or other surface) you are printing on also effects the useable resolution. For example, prints on canvas are often made at a lower resolution.

The table below shows how many pixels you need to print at a certain size at either 300 dpi or 240 dpi.

			Required image size
Target print size in inches			300dpi		240dpi
Short Edge	Long Edge		Short Edge	Long Edge	Short Edge	Long Edge
4	6		1200	1800	960	1440
5	7		1500	2100	1200	1680
8	10		2400	3000	1920	2400
8	12		2400	3600	1920	2880
8.3	11.7	(A4)	2490	3510	1992	2808
11	14		3300	4200	2640	3360
11.7	16.5	(A3)	3510	4950	2808	3960
12	16		3600	4800	2880	3840
16	20		4800	6000	3840	4800
20	30		6000	9000	4800	7200
24	36		7200	10800	5760	8640
30	40		9000	12000	7200	9600
36	48		10800	14400	8640	11520
40	60		12000	18000	9600	14400

Here are a few more examples taken from the table above.
Suppose you have a 12megapixel digital compact with sensor dimensions of 3000 * 4000 pixels.

According to the table, you could print up to A3, using a resolution of 240dpi, because this size would need 2808 * 3840 pixels which is slightly less than the resolution of your camera.

I say “according to the table” because I can’t predict whether the resulting print would meet your expectations for image quality. The requirements for hanging in an exhibition may be more exacting than for a print stuck to your fridge. Also, the resolution alone doesn’t tell us whether the camera is a high-end expensive device or a sub-£100 item.  Really, you just have to try it with your camera and see how it looks; but I think you can use the table to rule out extremes and note that you’re unlikely to get an acceptable 40″ * 60″ print from an 8 megapixel camera.

If you’re using GIMP and would rather not have to refer back to this table, then GIMP can give you the same information.

The “Print Size” option on the “Image” menu shows this dialog box:

The image used was a 34 megapixel file created by scanning medium format film at 2400dpi. Changing the units for width and height from pixels millimetres to inches, and entering 16 for width and 20 for height shows this display:

You can see that this image could be printed at  16 * 20 inches (a standard print size) at a resolution of over 300dpi.

And finally

It is becoming increasingly common for cameras to offer alternative aspect rations as well as their “native” aspect ratio. For example a camera with a 3:2 aspect ratio may have menu options that allow you to change to 4:3, 16:9, or 1:1 – with some reduction in total resolution as the cameras won’t use the pixels on the sensor which fall outside of the optional aspect ratios.

This feature may well be useful to you if you prefer an alternative aspect ratio and you want to be able to view the effect on screen rather than cropping on the computer.